Can i be written up for pda without a written warning?

I kissed my girlfriend bye today after school(just a peck), i told her to have a good weekend and to text me when she got home. A teacher turned around at the bus stop and saw us and said she was writing us up. I have never gotten a written warning or verbal warning on the subject. Can they do that? It was just us two and my friend, no one else around so it can't be "disruptive behavior" either. Please answer!:0